Mach-Zehnder modulators (MZI Mach-Zehnder interferometers) are used in optical transmission systems whose data rates are 10 Gbit/s or more since, at these high data rates and with the present-day state of the art, neither direct modulation of a laser nor modulation using electroabsorption modulators is expedient. Generally, in addition to the modulation signal, MZI modulators require a bias voltage for setting the operating point in order to achieve a balanced output signal and, hence, a balanced eye shape for the received signal. Any deviation of the operating point from this value leads to distortion in the optical transmission signal and, hence, to greater error rates and/or to a reduced range. Like all interferometer arrangements, which react extremely sensitively to a very small optical path length change, the operating point also varies with the environmental conditions, in most available modulators.
An MZI modulator is described, for example, in the xe2x80x9cDesigner""s Guide to External Modulationxe2x80x9d, UTP, 1289 Blue Hills Avenue, Bloomfield, Conn., pages 4-6. When the modulator is being fully driven, any change in the operating point leads to overdriving, as a result of which, after initially rising, the optical power falls once again, despite the control signal increasing, during the transition from blocking after switching on from xe2x80x9c0xe2x80x9d to xe2x80x9c1xe2x80x9d. In the event of overdriving, a transmitted xe2x80x9c1 bitxe2x80x9d peaks in the region of the rising 1 flank, resulting in a relatively high frequency structure in which some of the spectral power is contained in higher frequencies in particular; for example, at twice the fundamental frequency of the data signal, or at the data rate.
Patent specification U.S. Pat. No. 5,710,653 discloses a transmission system having a module for external modulation of a signal, in which harmonic frequencies of the modulated signal are suppressed. A first method uses two Mach-Zender interferometers, arranged in parallel, as modulators, to whose inputs the signal to be modulated is supplied with different amplitudes (80% and 20%), and whose two modulated output signals are combined such that a harmonic frequency from the second output signal is isolated from the fundamental frequency, is inverted and then amplified such that the harmonic frequencies in the resultant modulated signal compensate for one another by addition between the first modulated output signal and the processed isolated harmonic frequency of the second modulated output signal. In the second method, only one modulator is used in order to suppress second-order and third-order harmonic frequencies in the modulated signal. In this case, a distortion network is required to suppress the third-order harmonic frequency in addition to controlling the operating point of the modulator. These two methods suppress third-order harmonic frequencies. An additional control loop is provided for setting the operating point of the modulator or modulators in phase quadrature in order to eliminate the second-order harmonic frequency.
Patent Specification U.S. Pat. No. 5,629,792 discloses a further arrangement and method for modulation of a signal, with closed-loop control of the operating point of the modulator. The closed-loop control controls the operating point of the modulator by measuring the power level of the fundamental frequency of the modulated signal emitted by the modulator. This closed-loop process does not overcome, nor does it minimize, the influences of interference harmonic frequencies in the modulated signal.
An object of the present invention is, therefore, to specify a method which provides as simple a solution approach as possible for suppression of harmonic frequencies. Another aim is to specify a suitable system for doing the same.
Additional features and advantages of the present invention are described in, and will be apparent from, the following Detailed Description of the Invention and the Figures.
This object is achieved by using the signal at the harmonic frequency which, in each case, has been filtered out to derive a control signal which controls the operating point of the modulator, such that the at least one harmonic frequency reaches a minimum amplitude or power.
The advantage of this solution is that it requires considerably less complexity than the prior art for suppressing harmonic frequencies.
The method makes use of an effect, which occurs when the modulator is overdriven, for regulating the operating point. It is particularly advantageous in this case to combine operating point control with control of the modulation signal. All major parameters are kept constant by the control process.
It is also advantageous for the control criteria to be obtained at the receiving end, and to be transmitted via a service channel. This also makes it possible to partially compensate for distortion caused by the transmission path.
Additional features and advantages of the present invention are described in, and will be apparent from, the following Detailed Description of the Invention and the Figures.